Stroke is the 3rd largest cause of death and the largest cause of disability in the U.S., yet there are no effective therapies for the vast majorityof cases. Present therapeutic options merely aim to restore blood flow in the hopes of salvaging at risk tissue, but offer no targeted neuroprotection. Development of neuroprotective therapies has been hindered by lack of knowledge of the signaling pathways critical in secondary injury following ischemic stroke. For more than a decade, the caspase family of death proteases has been implicated in cerebral ischemia and neurodegeneration. Recent evidence shows that distinct caspase pathways are activated during ischemia. We have identified the caspase-9/-6 pathway as responsible for neuronal dysfunction and death after ischemia. Our data show that targeting caspase-9 activity provides substantial neuroprotection following an ischemic insult. Moreover, we find that caspase-9 activity is required for two aspects of ischemic pathogenesis: 1) neuronal degeneration and 2) the development of cerebral edema. Edema is caused by a loss of vascular integrity, rather than death of endothelial cells and pericytes in the blood vessels (BV). The elimination of tight junctions between these cells allows extravasation of fluid from small intracranial BVs. Edema formation is a major contributor to death and disability in severe stroke. Medical therapies and surgical decompressive procedures have only minimally altered the natural history of this pathogenic process. In our studies, a cell permeant caspase-9 inhibitor, Pen1-XBIR3, reduces caspase-9 activity and concomitantly abolishes edema. This finding opens the question of whether caspase-9 activity is a direct cause of edema through the impairment of vascular integrity of small cerebral BVs. Our preliminary data suggest that active caspase-9 regulates edema by decreasing the expression of matrix metalloproteinase 9 (MMP-9). Our data also show that expression of the precursor of mature NGF, proNGF, increases during stroke. ProNGF is a high affinity ligand for p75NTR, and we have shown that signaling through p75NTR activates caspase-9. p75NTR is found in small BVs in the brain, and expression of p75NTR increases during stroke. Our preliminary data also show that activated caspase-9 is present in BVs, and that caspase-9 inhibition prevents the stroke-induced expression of MMP-9. We now propose the hypothesis that the development of edema in stroke is mediated by proneurotrophin (proNT) signaling through p75NTR, which activates caspase-9 in small BVs to cleave substrates vital to the integrity of the vessels. We will utilize in vivo an in vitro models to examine this hypothesis with the following Specific Aims: Aim 1: To determine if induction of proNTs triggers caspase-9 activation in small BVs. Aim 2: To determine if signaling via p75NTR activates caspase-9 and leads to edema. Aim 3: To determine how caspase-9 cleavage of substrates leads to loss of vascular integrity.